This invention relates to the moulding of glass articles, and is concerned in particular with processes and apparatus for moulding glass articles by press moulding.
In known press moulding apparatus in current use for moulding for example glass ophthalmic lens blanks, it is common to use a press table rotating about a vertical axis and carrying a number, say six or eight, bottom mould parts spaced around its circumference for presentation in succession at a moulding station by rotary indexing movement of the table. Prior to the moulding station, a gob of molten glass is run into each bottom mould part, shortly before a top mould part is brought down at the moulding station to engage the bottom mould part and press the gob into shape.
Following pressing of an individual gob into the shape, the bottom mould part in which the moulded blank lies, is rotated on at intervals by successive indexing movements of the table, through a number of index positions in which the moulded blank is simply lying in the bottom mould part and cooling towards a temperature at which, at a pick-off station, the blank can be removed safely from the mould by a pick-off device, for example a suction head, and transferred to an annealing lehr. For example the average viscosity of the glass before moulding may be about 5 .times. 10.sup.3 poises corresponding to a glass temperature of 1050.degree.C in the case of a typical ophthalmic spectacle glass and before the blanks can be picked-off in the known apparatus the glass must cool to increase the viscosity to about 10.sup.10.5 poises, i.e. a temperature about 600.degree.C. For a given maximum indexing speed of the table, there is a maximum blank weight determined by these thermal requirements. Any blank with a weight exceeding this maximum cannot be processed unless the indexing speed is reduced below the maximum, with a consequent limitation on output. This can only be overcome by increasing the number of moulds employed with a consequent increase in capital and operating costs.
Other problems arise in connection with the control of mould temperature. To avoid sticking, it is desirable to control the temperature of the bottom plungers in the lower mould parts to stop their temperature rising too high. To avoid edge cracks the temperature of the bottom mould walls must be kept up. Apart from the inherent difficulties of providing control of these temperatures on a number of bottom mould parts on a continuously rotating table, further difficulties arise in that the mould temperatures must be allowed to vary as the table rotates to permit the blanks to cool, so that there are no set temperatures towards which the temperature of the different parts of the mould can be controlled.
Other operating disadvantages of the known apparatus are the initial cost and the replacement cost of the several lower mould parts, the production loss during the warming-up time required on starting-up before stable operating conditions are achieved, and the production loss or "down-time" incurred when replacing worn moulds or changing from one form of a moulded product to another, for example difficulty shaped ophthalmic lens blanks as every one of several lower mould parts has to be replaced or adjusted.
It is a main object of the present invention to provide an improved method and apparatus for the manufacture of moulded glass articles, in particular moulded lens blanks, which overcome the above mentioned disadvantages.